N-parallel-path capacitive switched filter in which only incomplete spaced portions of input signal are sampled

ABSTRACT

The invention provides a band-stop filter including a plurality n of switchably selectable capacitive parallel paths. Switch means are provided for sequentially and selectively selecting the paths, the switch means being controlled to select each path for a period of time which is less than 1/nth (usually 1/4n) of the period of a complete switching sequence of all of the paths.

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[56] References Eited UNITED STATES PATENTS 2,584,986 2/1952 Clark333/70 AU OTHER REFERENCES LePage et al. Analysis of a Comb Lifter Usingsynchronously Conimutated Capacitors, AIEE, March I953 333-7OA PrimaryExaminer llerman Karl Saalbach Assistant Examiner-Paul L. GenslerAttorneylBaldwin, Wight & Brown 'll'lltlliC'l: The invention provides aband-stop filter including a plurality n of switchably selectablecapacitive parallel paths. Switch means are provided for sequentiallyand selectively selecting the paths, the switch means being controlledto select each path for a period. of time which is less than l/nth(usually 1/411) of the period of a complete switching sequence of all ofthe paths.

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SHEET s or 8 LOW PASS F/LTER v) 8 5 I\ E R i PASS BAND STOP FREQUENCYBAND LOW 77 PASS FILTER m INVE TORS B) aWU/MM ZZ A Afifi ATTORNEYSPATENTEB SEP "/1911 3593 9 sum 5 BF 8 VOLTAGE ACROSS INV NTORS PATENTEDSEP Hen 3.603; 898 SHEET a nr 8 INVENTORS BY MW WWW MANOR 13g:

N-WilliALlLlEIL-FAIII CAFA CITIVlE SWII'IICIIIIIEID FIllL'IlEIIi INWllIlltCIll UhllL'I! llhltJtIh/IIPLIE'IIIE SF/tClED IPQIIITIIINS OF IlIlPlU'll I BIGNAIL Mill IiAilt/IIPILIEID This invention relates tofilter networks and more particularly to N-path band-stop filternetworks.

The invention is explained by reference to the accompany ing drawings inwhich:

FIG. I is a schematic diagram of a typical known N-path band-stop filternetwork; and

FIGS. 2 to 6 are explanatory diagrams relating thereto;

FIGS. 7 to 112 are explanatory diagrams relating to N-path band stopfilters in accordance with the present invention;

FIG. I3 is a schematic diagram of a modification of an N- path band stopfilter in accordance with the present invention; and

FIGS. I l, 15 and I6 are explanatory diagrams relating thereto;

FIG. I7 illustrates a further modification;

FIG. lid is a schematic diagram of another modification of an N-pathband stop filter in accordance with the present invention; and

FIG. I9 is an explanatory diagram relating thereto.

Referring to FIG. II, the known N-path band-stop filter network thereinrepresented consists of a number n of capacitors ll, 2, 3, 4i each inseries with a switch 5, s, 7 or d to form n switchably selectablecapacitive paths connected in parallel in one arm of the network betweeninput terminals 9, l and output terminals 11, 12. Each capacitor 1, 2, 3or t may be associated, by closing the respective switch 5, s, 7 or 8with a common shunt resistor I3 to form a CR filter. Between the CRfilter so formed and the output terminal III is connected a low passfilter M having no appreciable loading effect on the previous circuit.

The switches S, 6, 7 and 8 are closed sequentially and in turn by meansnot shown, the switch 6 closing as the switch 5 opens, the switch 7closing as the switch 6 opens and so on. In other words each switch isclosed for a period equal to l/nth of the period of a complete switchingsequence, where n is the number of switchable capacitive paths. This isillustrated in FIG. 2 in which the excursions from the base line S5represent times during which switch 5 of FIG. I is closed; theexcursions from the base line S6 represent time during which switch s ofFIG. I is closed; the excursions from the base line S7 represent timesduring which switch '7 of FIG. I is closed; and the excur sions from thebase line S8 represent times during which switch II of FIG. I is closed.Ifa sinusoidal input as represented in FIG. SI having a period IIT,where dT (see FIG. 2) is the period of a complete switch sequence of theswitches 5, a, 7 and ii, is applied to the input terminals 9, III) ofFIG. ll, switch 5 will sample the portions of the sinusoidal waveformshown shaded in FIG. 3 and of duration T; switch a will sample thefollowing waveform portions of duration T; and so on. It will be seenthat no charge accumulates in the capacitors and the whole of thesinusoidal wave appears as output across resistor I3.

If, as shown in FIG. I, a sinusoidal input having a period tlT isapplied to the input each capacitor has applied to it a voltage similarin magnitude and sign at each sampling and thus charges up to theaverage of the sinusoidal input during the time its switch is closed.The output across resistor I3 is thus reduced. FIG. 5 shows the outputacross resistor 113 under these conditions when switch 5 is closed. Theoverall response is a series of notches, as shown in FIG. 6, occurringat zero frequency and harmonics of frequency 1/41. The shape of the CRfilter provided by each of the I i-paths is reproduced on either side ofthe center rejection frequencies. The effect of the low pass filter M isshown in dashed line. The notch bandwidth at 3 db. attenuation is l/NrrrCR.

Except at zero frequency, where the notch depth is infinite, the notchdepth is limited since there is still a considerable output at thesynchronous frequency 1/41.

The object of the present invention is to provide improved N-path bandstop filter arrangements of increased notch depth.

According to this invention a band-stop filter including a plurality nof switchably selectable capacitive parallel paths and means forsequentially and repeatedly selecting said paths comprises means forselecting each path for a period of time which is less than l/nth of theperiod of a complete switching sequence of all of said paths.

The important result achieved by the invention is that there are duringthe switching sequence period, periods during which none of the pathsare selected to be in circuit.

In a preferred embodiment of the invention, each path is selected for aperiod of time which is equal to l/4n of the period ofa completeswitching sequence of all of said paths.

Preferably the band-stop filter is provided with sampling and holdingmeans connected and arranged to hold the output signal of a selectedpath substantially level during the period which follows the switchingoff of that path and during which none of the paths are selected.

Preferably the sampling and holding means is included between a commonshunt resistor at the output ends of said paths and a low pass furtherfilter through which output from the band-stop filter is taken.

Preferably the sampling and holding means consists of a series connectedswitch which, when closed, applies output signals from said paths tocharge a shunt connected capacitor, said series connected switch beingso controlled as to be closed during periods in which any of the pathsis selected and open during periods in which none of said paths isselected.

Preferably again the control of the series connected switch is such thatit is closed after a short delay following the selection of each path.This short delay may conveniently be one quarter ofthe time periodduring which the path is selected.

Preferably again the control of the series connected switch is such thatit is opened at approximately the middle of the time period during whicha path is selected. Preferably means are provided for adjusting the timeof opening of the said series connected switch.

In another way of carrying out the invention the band stop filter isprovided with sampling and holding means which precede the plurality ofswitching selectable capacitive parallel paths, and are adapted andarranged to sample an input waveform prior to a path being selected andto hold the samples value thereof substantially level during the periodin which said path is selected.

Preferably the said last mentioned sampling and holding means isarranged to sample the input waveform during each period in which noneof the paths are selected and to hold the samples value substantiallylevel between consecutive samples.

Preferably also said last mentioned sampling and holding means comprisesa series connected switch which, when closed, applies the input waveformto charge a shunt connected capacitor which is connected to applyvoltage thereacross to said paths.

Referring to FIGS. 7 to I7 illustrating the present invention, theoryshows that the operation of an Npath band'stop filter is only asdescribed with reference to FIGS. I to t for input frequencies of up tol/2T. For ease of explanation of the embodiments of the invention now tobe described, a 4i path band-stop filter network will be assumed and theexplanation will be concerned only with the notches at zero frequencyand 1 /4T. As mentioned hereinbefore the notch at zero frequency isinfinitely deep. The purpose of embodiments of the present invention nowto be described is, in all cases, to increase the depth of the notch atI MT.

In accordance with this invention the switches 5, s, 7 and II of a bandstop filter which, regarded purely as a circuit, is like that of FIG. I,are each closed (i.e. rendered conductive) for a period which is lessthan l/nth of the total switching sequence period IT in FIG. 2) where nis the number of paths, in this case I. In other words the switches 5,s, 7 and h of FIG. I are now so controlled by means (not shown) that, incontradistinction to a known band-stop filter as above-described withreference to FIGS. 1 to 6, all the switches are open (i.e.nonconductive) together for parts of the period following the closing ofone of the switches. In the specific example being described, eachswitch is closed for a period equal to l/l6th (i.e. l/4n) of the totalswitching sequence period. This is illustrated in FIG. 7 which, likeFIG. 2, shows base lines S5, S6, S7 and S8, the excursions from whichrepresent times during which the switches 5, 6, 7 and 8 are closed. Thecorresponding sampling of an input sinusoidal wave of period 4T by allthe switches is shown in FIG. 8, whilst the output due to the wave ofperiod 4T developed across resistor 13 is shown in FIG. 9. Thiscorresponds to a stop band. The corresponding sampling of an inputsinusoidal wave of period 8T by all the switches is shown in FIG. 10,whilst the output which is due to a wave of period ST and is developedacross resistor 13 is shown in FIG. 11. This corresponds to the passband. FIG. 12 shows the overall response. Again the dashed linerepresents the effect of the low pass filter 14.

It is obviously unnecessary to illustrate or describe in detail themeans for controlling the switches since such means may take any of anumber of suitable forms which will suggest themselves to those skilledin the art.

The output (at the input of the low pass filter) of the band stop filterabove described with reference to FIGS. 7 to 12 will be a series ofnarrow pulses within the pass band. These narrow pulses give, whenfiltered, an attenuated value of the input signal.

In the further embodiment of the invention illustrated in FIG. 13 alarger proportion of the value of the input signal may be obtained atthe output. The arrangement shown in FIG. 13 is generally similar tothat above described with reference to FIGS. 7 to 12 but with theaddition of a sampling and level holding circuit which precedes theinput to the low pass filter 14 and which consists of a switch 15, acapacitor 16, and a buffer stage 17. This sampling and holding circuitis operated to hold substantially constant the level of a pulse appliedto resistor 13 whilst the switches 5, 6, 7 and 8 are open. FIG. 14illustrates the operation of 15. As with FIG. 7, excursions from thebase lines S5, S6, S7, S8 and S15 of FIG. 14 represent the periods ofclosing of the switches 5, 6, 7, 8 and 15 respectively.

When switch 15 is closed capacitor 16 quickly charges up to the level ofthe signal pulse, and this level is held during the period in whichswitch 15 is open.

FIG. 15 shows a typical waveform for a signal in the pass band for oneperiod T following the closing of switch 15. Period T is the periodduring which the switch 15 is closed and capacitor 16 is charging up.Period T2 is the period during which switch 15 is open and the signallevel is held by capacitor 16.

FIG. 16 shows a typical waveform for a signal in the stop band for oneperiod T following the closing of switch 15. Again period T is theperiod during which switch 15 is closed and period T is the periodduring which switch 15 is open. T For a sample having a width T muchless than T, the line AB in FIG. 16 tends towards the linear and P, thepoint of zero output, becomes the center point. If, as illustrated inFIG. 17, switch 15 is controlled to be closed for a short period at orabout the point P the stop band attenuation is improved whilst the passband characteristic is maintained substantially unchanged.

The use of the sampling and holding circuit 15, 16 as described alsosimplifies the task of the low pass filter 14 of smoothing the outputsince, as will be appreciated, the output thus provided is much closerto the finally desired output than is an output consisting of a seriesof relatively short pulses.

Again the means (not shown for controlling the switching of switch 15may take any number of suitable forms, known per se.

The means controlling the opening and closing of the switch 15 may besuch that the point P, i.e. the time at which switch 15 opens, isadjustable so as to permit the maximum attenuation at the notch center,or at a nearby frequency, to be obtained.

Providing an adjustable delay before switch 15 is closed and providingmeans for adjusting the duration of the signal which controls switch 15are two ways of achieving this. Again the practical implementation ofthis feature of the present invention is within the competence of theskilled man and detailed description or illustration is unnecessary.

The filter arrangement in FIG. 18 generally similar to that describedwith reference to FIGS. 7 to 12 but with the addition in front of theband-stop filter of a sampling and level holding circuit consisting ofaswitch 18 and a capacitor 19. As will be seen a buffer stage 20 isinterposed between the sampling and holding circuit 18, I9 and theN-path band stop filter. The operation of the arrangement of FIG. 18 isexplained with reference to FIG. 19 in which:

a. shows a sinusoidal input wave of frequcncyf=( l/4T) b. shows thetimes of closing of switch 18 as excursions from a base line S18;

0. shows the charge upon capacitor 19 as a result of sampling the inputwave (a) by switch 18; and

d. shows the times of closing of switches 5, 6, 7 and 8 as excursionsfrom base lines S5, S6, S7 and S8 respectively.

As will be seen switch 18 samples the input wave for short periods andthe voltage present is held by capacitor 19, resulting in asubstantially flat topped voltage waveform being present for sampling bythe switches 5, 6, 7 and 8. The switches 5, 6, 7 and 8 are synchronizedto sample in the periods when switch 18 is open, i.e. on flat portionsof the voltage waveform (c). Thus there can be no appreciable change ofinput voltage whilst any of the switches 5, 6, 7 or 8 are sampling. Ator near the frequencyf=( l/4T) the average value of the input duringsampling by switches 5, 6, 7 and 8 is removed and the portion of theinput wave e the low pass filter 14 is reduced, thus tending furtherincreasing the notch depth at l/4T.

We claim:

1. A band-stop filter including a plurality n of switchably selectablecapacitive parallel paths, means for sequentially and repeatedlyselecting said paths wherein each path is selected for a period of timewhich is less than l/nth of the period of a complete switching sequenceof all of said paths, a common shunt impedance at the output end of saidpaths, and a low pass filter through which output from the band-stopfilter is taken.

2. A filter as claimed in claim 1 wherein each path is selected for aperiod of time which is equal to l/4n of the period of a completeswitching sequence of all of said paths.

3. A band-stop filter as claimed in claim 2 wherein there is providedsampling and holding means connected and arranged to hold the outputsignal of a selected path substantially level during the period whichfollows the switching off of that path and during which none of thepaths are selected.

4. A band-stop filter as claimed in claim 3 wherein the sampling andholding means is included between said common shunt impedance at theoutput ends of said paths and said low pass filter through which outputfrom the band-stop filter is taken, said shunt impedance being aresistor.

5. A band-stop filter as claimed in claim 3 wherein the sampling andholding means consists of a series connected switch which, when closed,applies output signals from said paths to charge a shunt connectedcapacitor, said series connected switch being so controlled as to beclosed during periods in which any of the paths is selected and openduring periods in which none of said paths is selected.

6. A band-stop filter as claimed in claim 5 wherein the control of theseries connected switch is such that it is closed after a short delayfollowing the selection of each path.

7. A band-stop filter as claimed in claim 6 wherein the delay issubstantially one quarter of the time period during which the path isselected.

ll. A band-stop filter as claimed in claim 5 wherein the control of theseries connected switch is such that it is opened at approximately themiddle of the time period during which a path is selected.

9. A band-stop filter as claimed in claim 5 wherein means are providedfor adjusting the time of opening of the said series connected switch.

110. A band-stop filter as claimed in claim ll wherein the band-stopfilter is provided with sampling and holding means which precede theplurality of switehably selectable capacitive parallel paths, and areadapted and arranged to sample an input waveform prior to a path beingselected and to hold the sampled value thereof substantially levelduring the period in

1. A band-stop filter including a plurality n of switchably selectablecapacitive parallel paths, means for sequentially and repeatedlyselecting said paths wherein each path Is selected for a period of timewhich is less than 1/nth of the period of a complete switching sequenceof all of said paths, a common shunt impedance at the output end of saidpaths, and a low pass filter through which output from the band-stopfilter is taken.
 2. A filter as claimed in claim 1 wherein each path isselected for a period of time which is equal to 1/4n of the period of acomplete switching sequence of all of said paths.
 3. A band-stop filteras claimed in claim 2 wherein there is provided sampling and holdingmeans connected and arranged to hold the output signal of a selectedpath substantially level during the period which follows the switchingoff of that path and during which none of the paths are selected.
 4. Aband-stop filter as claimed in claim 3 wherein the sampling and holdingmeans is included between said common shunt impedance at the output endsof said paths and said low pass filter through which output from theband-stop filter is taken, said shunt impedance being a resistor.
 5. Aband-stop filter as claimed in claim 3 wherein the sampling and holdingmeans consists of a series connected switch which, when closed, appliesoutput signals from said paths to charge a shunt connected capacitor,said series connected switch being so controlled as to be closed duringperiods in which any of the paths is selected and open during periods inwhich none of said paths is selected.
 6. A band-stop filter as claimedin claim 5 wherein the control of the series connected switch is suchthat it is closed after a short delay following the selection of eachpath.
 7. A band-stop filter as claimed in claim 6 wherein the delay issubstantially one quarter of the time period during which the path isselected.
 8. A band-stop filter as claimed in claim 5 wherein thecontrol of the series connected switch is such that it is opened atapproximately the middle of the time period during which a path isselected.
 9. A band-stop filter as claimed in claim 5 wherein means areprovided for adjusting the time of opening of the said series connectedswitch.
 10. A band-stop filter as claimed in claim 1 wherein theband-stop filter is provided with sampling and holding means whichprecede the plurality of switchably selectable capacitive parallelpaths, and are adapted and arranged to sample an input waveform prior toa path being selected and to hold the sampled value thereofsubstantially level during the period in which said path is selected.11. A band-stop filter as claimed in claim 10 wherein the sampling andholding means is arranged to sample the input waveform during eachperiod in which none of the paths are selected and to hold the samplesvalue substantially level between consecutive samples.
 12. A band-stopfilter as claimed in claim 10 wherein the sampling and holding meanscomprises a series connected switch which when closed, applies the inputwaveform to charge a shunt connected capacitor which is connected toapply voltage thereacross to said paths.